Many data processing systems require a large amount of data storage, for use in efficiently accessing, modifying, and re-storing data. Data storage is typically separated into several different levels, each level exhibiting a different data access time or data storage cost. A first, or highest level of data storage involves electronic memory, usually dynamic or static random access memory (DRAM or SRAM). Electronic memories take the form of semiconductor integrated circuits where millions of bytes of data can be stored on each circuit, with access to such bytes of data measured in nanoseconds. The electronic memory provides the fastest access to data since access is entirely electronic.
A second level of data storage usually involves direct access storage devices (DASD). DASDs can store gigabytes of data, and the access to such data is typically measured in milliseconds, i.e. orders of magnitudes slower than electronic memory. Often, this second level of data storage is performed by a data storage and retrieval system interconnected to one or more host computer systems.
Such data storage and retrieval systems receive requests to write information to one or more data storage devices, and requests to retrieve information from those one or more data storage devices. Upon receipt of a write request, the system stores information received from a host computer in a data cache. In certain implementations, a copy of that information is also stored in a nonvolatile storage device. Upon receipt of a read request, the system recalls information from the one or more data storage devices and moves that information to the data cache. Thus, the system is continuously moving information to and from a plurality of data storage devices, and to and from the data cache.
Having a backup data copy is mandatory for many businesses for which data loss would be catastrophic. Primary data is periodically provided from a primary information storage and retrieval system to a secondary information storage and retrieval system. Such primary data is backed-up by making a copy on, for example, tape or library storage at the remote storage location.